The present invention relates to an acoustic surface wave device which serves as a band-pass filter, a resonator, a delay line or the like.
In general, conversion of acoustic surface waves propagating across a piezoelectric substrate to electrical energy and vice versa is carried out by a transducer which is conventionally composed of a pair of separated, interdigitated comb-shaped electrodes or a pair of separated, venetian blind shaped electrodes formed on the surface of the piezoelectric substrate.
One conventional acoustic surface wave device comprises: a piezoelectric substrate made of lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3) or the like, having a plane surface for the propagation of acoustic surfaces waves; and input and output transducers formed on the surface of the substrate. When an electrical signal is applied to the input transducer, the electrical signal is converted into acoustic surfaces waves which are propagated to the output transducer. As a result, the acoustic surface waves are reconverted into other types of electrical signals by the output transducer. In this case, several kinds of waves are generated, as follows:
(a) desired acoustic surface waves which are launched by the input transducer and are propagated on the surface of the substrate to reach the output transducer; PA1 (b) undesired bulk waves which are launched by the input transducer and are propagated through the body of the substrate to reach the output transducer; PA1 (c) undesired surface wave reflections which are launched by the input transducer and are reflected by the output transducer and, in addition, are reflected by the input transducer to reach the output transducer; PA1 (d) undesired surface wave reflections and bulk waves which are launched by the input transducer and reflected by the end face of the substrate to return to the input transducer and, in addition, to reach the output transducer; and PA1 (e) undesired surface reflections and bulk waves which are launched by the output transducer and are reflected by the end surface of the substrate to return to the output transducer.
In particular, the reflections explained in item (c) are called triple transit echos (hereinafter referred to as TTEs), since the reflections travel three times between the input and output transducers. The undesired surface wave reflections and bulk waves explained in items (b) through (e), above, deteriorate the band-pass (filter) characteristics, since their transit times are different from that of the desired acoustic surface waves explained in item (a).
In order to reduce undesired bulk waves and TTEs, explained in items (b) and (c), above, respectively, a multistrip coupler (hereinafter referred to as an MSC), which is composed of a plurality of parallel and equally spaced conductors, is conventionally formed on the surface of the substrate and interposed between the input and output transducers. In this case, the input and output transducers are located diagonally with respect to each other. The MSC transfers the propagation path of acoustic surface waves from one track to another track. In other words, the MSC serves as a path changer only for acoustic surface waves, not for bulk waves which travel through the body of the substrate. Therefore, bulk waves do not reach the output transducer. Note that the MSC can also reduce the TTEs explained in item (c), above.
On the other hand, in order to reduce undesired surface wave reflections and bulk waves, explained in items (d) and (e), above, acoustical absorbent layer patterns made of oleoresin or urearesin are conventionally formed on the substrate behind the input and output transducers. The acoustical absorbent layer patterns absorb most acoustic surface waves and bulk waves, but reflect some of the acoustic surface waves and bulk waves. In this case, the acoustic absorbent layer patterns have zigzag-shaped edges facing the input or output transducer so as to change the reflection path direction of the undesired waves, thereby preventing the undesired waves from being propagated into the input or output transducer.
In this case, however, since the acoustical absorbent layer patterns are conventionally coated by the screen print method the patterns sometimes spread due to the change in viscosity. As a result, the tip of each recess portion of the acoustical absorbent layer patterns becomes rounded with the result that it becomes parallel to the wave front of the acoustic surface waves. Therefore, the reflections from the tip of each recess portion return to the input or output transducer, which causes the problem of degration in the band-pass characteristics.